captures two stones. Black can play
at
and recapture a stone. This is not a ko. Also, this sequence gives no eye to White.
There are a lot of situations where this sequence is a possible variation, the consequences of which are easily overlooked. An example:
Black to live. The combination of the marked shape with the 1-2 point is very common in life and death problems. Yet our desire to atari may overcome us:
This is a line easily visualized and often played by both players if they fail to see the meaning of this shape.
White plays and recaptures a stone with
at
after
takes two stones. Black is dead.
Black's correct move is the 1-2 point. White can't play at a due to shortage of liberties. If White next b, Black a gives life.
Dieter (maybe this material has already been covered elsewhere)
See also: